The following section describes GPS technology at an overview level. It is intended to give readers who are unfamiliar with GPS or just need a brief refresher, a thumbnail sketch of the technology's basic functionality and its intended markets, as well as an understanding of its relevant implementation and political issues. For more detailed information on such topics as GPS architecture and its associated standards, please investigate the resource links on this page.
The Global Positioning System (GPS) was originally designed and deployed as a military tool. GPS uses satellite technology to pinpoint an exact location on the planet. It does this by using a minimum of three satellites transmitting a signal to an earth based receiver. The receiver requires a minimum of three satellite signals to trilaterate the position (similar concept to triangulation). It does this based on the discrete travel times of the radio signals emanating from the different satellites to the receiver.
Its early use was for military navigational and/or targeting purposes. GPS has proven to be exceptionally useful in maritime and aviation applications, where one’s landmarks can be visually challenging.
In the consumer electronics arena, GPS has gained in popular use in automotive navigation (e.g., Onstar) and off-road leisure activities such as hiking, fishing etc. And, in emergency situations, the GPS components in cell phones can be used to find someone lost and/or injured.
There are a number of business applications in use today. For example, land survey, avionics, supply chain shipment
tracking, and GPS is now being incorporated into earth moving equipment for farming, mining, and waste handling.
The GPS concept originated during the race to space between Russia and the United States. U.S. scientists realized that they could monitor Sputnik’s transmissions and determine its position in the sky by measuring the Doppler distortion of the signal’s frequency between the satellite and their known position on earth. They realized that the converse would also be true that if the satellites position was “known” then they could determine a particular location on earth.
GPS satellites were first launched over 20 years ago in 1978, paid for by the American taxpayer. However, it was not until 1993, when a full constellation of 24 satellites were deployed, that it was considered fully operational. Early commercial applications in 1984 were ascertaining position fixes on offshore oil rigs, and surveying, when GPS equipment was very expensive ($150K) as well as large and unwieldy.
Handheld units arrived on the scene in 1989 and their purchase price was $3,000 - still not in the price range for “casual” hobbyist users. However, by 1995 that barrier was crossed when handhelds came down to $200 a unit, making it feasible for hunters, fishermen and hikers. Now GPS is integrated into your cell phones, even though in most cases it has not been activated by the service provider. CDMA cell sites use GPS for network synchronization. As a result of the current price point of GPS receivers and increasing accuracy, GPS is showing up in many new personal and business applications.
For national security reasons, the civilian signaled was originally deliberately injected with an error factor, referred to as selective availability (SA). In May 2000, the government turned off SA. It should be noted that the military can “jam” GPS signals over a particular geographic region if necessary for national security purposes.
The architectural components of GPS are typically referred to as the control segment (ground stations), the space segment
(satellites) and the user segment (receivers).
Control Segment – Ground Stations: There are six operational control system (OCS) monitoring stations and four ground antenna stations. These stations track all GPS signals. Three of them are capable of uplinking to the satellites. In other words, they both speak and listen to the satellites, updating them with regard to clock corrections and satellite positions. They listen to the satellites to determine their “health” by looking at their signal integrity and orbital position stability. These ground control stations are all under the jurisdiction of the U.S. Department of Defense (DoD) and are positioned around the world. The master station is located in Colorado Springs at the Air Force base. The master station can send commands to the satellites to make orbit adjustments, upload new software and so on.
In 2005 additional feeds from an initial set of six National Geospatial-Intelligence Agency (NGA) stations were included in the OCS data processing. And the OCS data modeling was improved to be able to better use these additional data feeds.
Space Segment – Satellites: There are 29 U.S. GPS satellites in orbit (some are spares). A minimum of 24 active satellites is required to be fully functional. These satellites are constantly streaming data over a downlink. Their signals can be read by GPS receivers anywhere in the world. However, the receiver must have a minimum of four satellites “in view.” Buildings, terrain and electronic interference can block signal reception. The satellites vary in age and have a lifespan of seven to twelve years and have to be replaced as they “expire.” Future satellites will offer additional civilian signals.
The progression of satellite series currently operational starts with 16 II/IIA (in Block II), followed by 12 IIR. Most recently two of IIR-M series were launched.
User Segment – Receivers: These receivers read the available satellite signals to determine a user’s position, velocity and time. Underlying how these components work together is the clock and the satellites’ orbit. To get an accurate position fix, a receiver “sees” at least four satellites. The receiver uses the time stamp from the satellite to determine the transmission delay. Getting this information simultaneously from a minimum of four different satellites is what enables the calculation of a user’s 3-D position. Position refers to the coordinates in 3-D space – in other words, not just where one is standing, but how high. 2-D (latitude and longitude) can be determined with only 3 satellite signals.
Determining Position: Upon taking in all available satellite signals, the receiver compares the time that the satellite sent the signal to the time it was received for each of the available signals. Trilateralization (similar to triangulation) then calculates the position by comparing the difference among the signals. See the figure below for an illustration in a simplified 2-D view.
Accuracy of GPS: The accuracy of GPS depends on a number of factors, number of channels on the receiver, number of satellites in view, and signal interference caused by buildings, mountains and ionospheric disturbances. Accuracy should be within 15 meters (without SA) provided the receiver has a clear shot at a minimum of four satellites.
There are several methods that can improve GPS accuracy. Two commonly discussed are Differential GPS (DGPS) and Wide Area Augmentations System (WAAS). These improve accuracy to within 1 to 3 meters. DGPS uses fixed, mounted GPS receivers to calculate the difference between their actual known position and the calculated GPS position. This difference is then broadcast over a local FM signal. GPS units within range of the local FM signal can improve their position accuracy to within 1cm over short distances (but more typically 3-5 meters).
WAAS, developed and deployed by the FAA, takes this approach a step further. Today there are 25 WAAS ground stations networked together. These communicate errors back to the wide area master station. The master station applies correction algorithms to the original GPS data stream and sends a correction message to a geosynchronous satellite. This satellite then transmits on the same signal as GPS satellites. This correction results in better than a 3 meter degree of accuracy.
Coverage today for WAAS is continental United States and parts of Canada. There are variants in the works. For example, Europe is developing EGNOS, the Euro Geostationary Navigation Overlay Service, and in Japan, MSAS, the Multifunctional Satellite Augmentation System.
Position Fix vs. Waypoints: When you take a reading on GPS, it gives a real-time position fix. Should you decide to store that information so that you can return to that exact location, it can be saved in memory as a waypoint. Obviously, how many waypoints you can store depends how on much memory your receiver has available.
NMEA 0183 – a proprietary protocol National Marine Electronics Association, a U.S. standards committee, defines data message structure, contents and protocols to enable communication between the GPS receiver and other electronic components.
WGS-84 The World Geodetic System (WGS 84) datum is used to represent GPS coordinates. WGS 84 is specific to NAVSTAR. Other coordinate systems are ECEF – Earth Centered Earth Fixed Cartesian Coordinates and the familiar latitude, longitude and altitude.
There is a published defined signal structure and message format that was designed and developed for NAVSTAR (the U.S. GPS system).
Russia expects to have Glonass fully operational in 2008 (they have 13 satellites in operation now) and the European Union is targeting to have Galileo by 2010. The first Galileo satellite launch was in December 2005 and its second is scheduled to be launched by the end of 2006.
GIS and How It Relates to GPS
GIS stands for Geographic Information System. GIS is an open computer system designed to allow users to collect, manage, communicate and analyze relevant spatial data of the earth. Obviously there is a natural relationship between GPS, which provides a mechanism to capture geographic positions in 3 dimensions, and mapping technology such as GIS.
New satellites, the IIR-M are being launched. The first went up in December 2005, and the next two are scheduled to go up in Sept ‘06 and Dec ‘06. These satellites will add a second civilian band signal (L2C) and the new M-Code, which eliminates SA for denial and also has anti-jam flex power. The next block of satellites IIF add support for a third civil signal (L5) and is often referred to as next generation GPS. There has now been a hold put on the procurement of Block IIIA satellites, although supposedly the target date of 2013 for the generation 3 satellites will still be met. These Block 3 satellites are supposed to be compatible with Galileo baseline L1 service signal.
With increased accuracy come new applications. An array of new applications will become feasible with the increased
positioning accuracy available from Next Generation GPS.